There are times when topnotch financial advice is a must

Most people can - or think they can - go through their entire working life without consulting a professional tax accountant, financial planner, lawyer, or other specialist to help them with their financial affairs.

But there is one time when an investment in one of these professionals is not only worthwhile, it may be essential. That is at retirement. The tax and financial planning consequences of a sudden infusion of money from a tax-deferred retirement plan, individual retirement account (IRA), Keogh plan, annuity, or profit-sharing and stock bonus plans are all different and complicated. Each has its own tax implications that can affect the ways an individual gets his or her money and how much of it he or she gets to keep.

One of the neatest ways to keep the tax man from putting too big a crack in your nest egg is a technique known as 10-year forward averaging. Unlike ordinary five-year income averaging, which only compares the incomes of a taxpayer's previous five years, 10-year forward averaging spreads a lump-sum distribution out over the next 10 years.

Ten-year averaging cannot be used with everything. It is available only for previously untaxed payments from salary reduction plans, Keoghs, company savings plans, employee stock ownership plans (ESOPs), profit-sharing plans, and tax-sheltered annuities held by teachers and employees of nonprofit organizations.

In an example provided by Howard Lane, senior trust officer at State Street Bank of Boston, a retiree receives a lump-sum payment of $52,250. The tax is computed as if the person were single and as if this were the only income he or she had, making a tax burden of $434.70 for each of those years. Multiplied times 10 years, this comes to a tax bill of $4,347.50, or just 7.7 percent of the original $52,250.

That $4,347.50 is all payable now, but it is certainly better than having to send the IRS as much as half of the original windfall.

However, there are some qualifiers on this trick, Mr. Lane points out. For example, you have to have participated in the tax-sheltered plan for at least five years; you can't have received the money before you are 59 1/2 years old; and you can only do it once after that age, so if you have several tax-deferred savings plans, you must put them together for one 10-year forward averaging exercise.

With some tax-sheltered plans, it is possible to choose between a lump-sum distribution and periodic ''annuitized'' payments. If an individual has a choice , he should do the paper work on both 10-year forward averaging and annuitizing and compare them.

If the second option is selected, the firm holding the retirement money - an insurance company, most likely - will figure the annual payments on the basis of currently accepted guidelines of life expectancy and on an interest rate the company chooses. This rate is usually a couple of percentage points lower than the one used when the account was building up before retirement.

Another way to handle at least part of a lump-sum distribution would be to put it in an IRA. Although you cannot make any payments to an IRA after age 70 1 /2 and payments from it must begin then, you have those 11 years starting at 59 1/2 when you can still make IRA contributions. If both a husband and wife are still earning income (even part-time or free-lance work will do), they can put up to $4,000 a year into the IRA.

An IRA, of course, can also be the source of the lump-sum withdrawal. If you were one of those who were able to contribute to an IRA before 1982, when they became available to all wage-earners, there may be $100,000 or so waiting for you. One way to defray the tax consequences of this windfall might be to spread it out among several single premium deferred annuities, or SPDAs. They should be purchased from several separate insurance companies to minimize your risk should any one company get into financial difficulty, as Baldwin-United did earlier this year.

Once you have your SPDAs, however, there are several payment options. With ''single life,'' you receive payments as long as you live, but there is nothing for your heirs. With ''life, years certain,'' you are guaranteed income for life. But if you do not live for very many years after retirement, payments continue to the heirs for 10 to 20 years.

With ''refund annuity,'' you or your beneficiary get payments until the amount paid out equals the original investment. Finally, ''joint and survivor'' provides income as long as you or your spouse live. The income you receive from all of these options includes both principal and interest.

There are also variable annuities where your return depends on the success of the investments in the portfolio. If the stocks in the portfolio climb in value, your payments will climb, too. But the stock market also has its down periods, which could cut payments a retiree may need.

Finally, there is another method for easing the burden of a lump-sum distribution available to people who contributed to their pension plan prior to 1974. A change in the tax laws in that year specified that these distributions would henceforth be subject to the 10-year forward averaging described earlier; money put in before then would be taxed at the even more advantageous capital-gains rate. So a person with 30 years of contributions to a pension plan and retiring this year could figure two-thirds of his distribution at the capital-gains rate. This provision applies even if considerably more than one-third of the money was built up in the 10 years since 1974, when the retiree may have been at the peak of earning power; or if the contribution has been made for only 20 years, the capital-gains rate would apply to one-half the lump-sum distribution.

Again, a tax expert or financial planner can more fully explain these ideas and suggest others, so this is not the time for do-it-yourselfers - unless you are an accountant or other tax professional - and even these often have one of their colleagues back them up when it comes to their own finances.